Protection from HIV was 60% at 12 months after immunizing low-risk volunteers with canarypox vCP1521 priming plus Envelope glycoprotein boosting (RV144 trial), but dropped to 29% after 42 months consistent with transient Env-specific antibodies (Kim, et al., 2010). Increasing the durability of protective immunity is a critcal objective for improving vaccines. Our goals are to identify defects in the immune response to antigen which block the normal pathways for generating B cell memory and fail to produce durable antibodies. We focus on CD4+/CXCR5+ T follicular helper cells (Tfh) which, together with antigen-specific B cells, normally initiate the germinal center reaction, promote affinity maturation of antibody and generate memory B cells. We postulate that Tfh are negatively impacted by Env and fail to support normal B cell development. Tfh have not been evaluated in HIV or SIV vaccine studies. They are poorly understood in nonhuman primates (NHP), where conditions for generating Tfh are unknown and functions of this subset have not been tested. Our efforts will relate Tfh to memory B cells in immunized NHP. Comparing animal groups that receive vector (canarypox), prime/boost (vCP1521 recombinant canarypox/Env glycoprotein), or Env alone, we ask whether Env has a negative effect on Tfh generation that is overcome by the vCP1521 priming immunization. Immunized macaques undergo repetitive, low-dose intrarectal virus challenge (as part of a funded preclinical vaccine study), allowing us to test whether Tfh and/or antibody-secreting memory B cells are correlates of protection that might be useful in human vaccine trials. Results from these studies will show whether transient antibody responses to vaccine reflect a fundamental defect in the pathway for memory B cell generation with Env antigens. Mitigation strategies might include modification of Env (changing protein or glycan structures), incorporation of cytokines, or vaccine formulations which add TLR7/8 agonists, all strategies to improve the Tfh response and memory B cell formation. There have been few efforts to determine mechanisms underlying the well-known transient antibody responses to Env glycoproteins. Our work concentrates on one aspect of antibody generation and tests its relationship to vaccine efficacy. PUBLIC HEALTH RELEVANCE: Current vaccines protect against HIV infection for around 1 year, but then begin to fail. Increasing the durability of protective immunity is key to obtaining vaccines which will impact public health. Our goals are to look for defects in the fundamental steps of generating long-term antibodies that have not been evaluated in previous animal or human vaccine studies. Results from this work may provide new approaches for early detection of protective antibody responses during vaccine trials and may guide the development of new vaccines with longer term protection against HIV.